There is a demand for highly reflective metal flake for many purposes, particularly in the production of paints, inks, and the like. One technique for producing metal flake involves a wet milling technique in which metal powder is ball milled in a low viscosity liquid, referred to herewith as a mobile liquid, the liquid eventually being driven off, e.g. by centrifuging and vacuum drying. The liquid is recovered and if in satisfactory form therefor it is reused. It is also known to incorporate a small amount of a lubricant such as fatty acids in the liquid medium to minimize or prevent welding of the particles being milled. U.S. Pat. Nos. 3,709,439 and 3,941,584, for example, disclose a method of making stain-resistant metals and alloys including nickel, which have smooth, specularly reflective surfaces, are bright and lustrous and are characterized by aesthetic effectiveness in paint films and other applications. U.S. Pat. Nos. 3,776,473 and 3,901,688 disclose a method for making highly reflective aluminum flake. Although these processes have been found very satisfactory for producing highly reflective metal flake, it would be an advantage to improve the reflectivity of the flake still further, e.g., for decorative purposes. Furthermore, it has been found that in the milling process there is a tendency to degeneration of the liquid medium, which limits its re-use and adds to the cost of processing.
It has now been found that with the use of a suitable lubricant in the mobile liquid system as a substitute for fatty acid lubricants, metal flake can be obtained which has improved reflectivity. It has also been found that with the lubricant of the present invention, the deterioration of the liquid medium is at least reduced considerably. Moreover, the flake is produced in accordance with this invention with high production efficiency and reproducibility.
These and other advantages of the present invention will become apparent from the description which follows taken in consideration with the examples. In this connection, it is noted that although the present invention is described herein with particular reference to nickel, the same principles apply to other conductive metals and alloys, as will be evident to those skilled in the art.